Gilbert mixers with improved isolation and methods therefor

ABSTRACT

The employment of cascoding in connection with improving mixer isolation in a Gilbert mixer circuit is provided. In this vein, there is broadly contemplated herein, inter alia, the provision of a mixer suitable for use in a direct-conversion radio receiver operating in the 57-64 GHz industrial, scientific, and medical (ISM) band. Such a receiver may be integrated along with a transmitter entirely on a silicon integrated circuit and can be used to receive and transmit data signals in such applications as wireless personal-area networks (WPANs). Numerous other applications, of course, are available for a mixer with improved LO-to-RF isolation, particularly at millimeter-wave frequencies where high LO-to-RF isolation is difficult to achieve.

FIELD OF THE INVENTION

The present invention relates generally to mixers used in radioreceivers and transmitters, and more specifically to methods forimproving isolation between the signal paths for a local oscillator (LO)and a radio frequency (RF) input or output signal.

BACKGROUND OF THE INVENTION

Herebelow, numerals in square brackets—[ ]—are keyed to the list ofreferences found towards the close of the present disclosure.

In the general realm of direct-conversion radio receivers operating athigh frequencies, it is well known that leakage of the local-oscillator(LO) signal into the RF-signal path can generate cross-modulation andinter-modulation distortion and time-varying DC offsets in the mixerstage of the receiver. Since the LO signal and the RF signal are at thesame frequency, the LO signal which leaks into the RF-signal path isindistinguishable from the RF signal itself, and the LO leakage is mixedwith the LO signal itself in the mixer to appear as a distortion ornoise component in the mixer's baseband output.

Accordingly, it has been found to be desirable to minimize the leakageof the LO signal into the RF-signal path. Conventionally, this has beendone at least in part by laying out the circuits so as to minimize thecoupling between the LO and RF signal paths. In the mixer itself,double-balanced mixers are preferred [1,2] since they provide generallygood isolation between the LO port and the RF-input port.

However, at sufficiently high frequencies, LO-to-RF isolation of even adouble-balanced mixer can become inadequate. Inherent imbalances in thecircuit due to imperfect component matching results in incompletecancellation of the LO fundamental at the common emitters of theswitching quad, and leakage of the LO fundamental frequency to the RFinput occurs through the base-collector junction capacitance of theRF-input transistors.

Accordingly, a need has been recognized in connection with improvingupon the shortcomings and disadvantages experienced with conventionalarrangements as discussed above.

SUMMARY OF THE INVENTION

There is broadly contemplated herein, in accordance with at least onepresently preferred embodiment of the present invention, the employmentof cascoding in connection with improving mixer isolation in a Gilbertmixer circuit.

In this vein, there is broadly contemplated herein, inter alia, theprovision of a mixer suitable for use in a direct-conversion radioreceiver operating in the 57-64 GHz industrial, scientific, and medical(ISM) band. Such a receiver may be integrated along with a transmitterentirely on a silicon integrated circuit and can be used to receive andtransmit data signals in such applications as wireless personal-areanetworks (WPANs). Numerous other applications, of course, are availablefor a mixer with improved LO-to-RF isolation, particularly atmillimeter-wave frequencies where high LO-to-RF isolation is difficultto achieve.

As a further notable advantage in connection with at least oneembodiment of the present invention, by employing internal cascodingbetween the switching quad and the transconductance stage, feedbackthrough the base-collector junction of the RF-input transistors can besignificantly reduced.

In summary, one aspect of the invention provides a Gilbert mixer circuitcomprising at least one cascode element for improving mixer isolation.

Another aspect of the invention provides a method of improving mixerisolation in a Gilbert mixer, the method comprising the step ofproviding at least one cascode element for improving mixer isolation.

For a better understanding of the present invention, together with otherand further features and advantages thereof, reference is made to thefollowing description, taken in conjunction with the accompanyingdrawings, and the scope of the invention will be pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a conventional Gilbert mixer circuit.

FIG. 2 schematically depicts a mixer circuit, in accordance with apreferred embodiment of the present invention, which includes cascodetransistors.

FIG. 3 schematically depicts a circuit similar to that in FIG. 2, butwith the inclusion of a parallel resonant LC circuit.

FIG. 4 schematically depicts a circuit similar to that in FIG. 3, but(for the purpose of experimentation) without cascode transistors.

FIG. 5 conveys results of a circuit simulation comparing cascoded andnon-cascoded mixers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a conventional double-balanced Gilbert mixer [1-2].The LO signal is applied to the bases of the switching quad (Q3-Q6). TheLO signal which appears at the two common emitters of the switching quad(that is, the common emitter of Q3 and Q4, and the common emitter of Q5and Q6) is primarily the second harmonic of the LO frequency. However,due to imperfect matching of Q3 and Q4 (and Q5-Q6), some fundamentalfrequency LO signal does appear at these nodes. The fundamentalfrequency LO signal can then leak back to the RF input by way of thebase-collector capacitance of Q1 and Q2.

In connection with a mixer such as that shown in FIG. 1, at mostfrequencies, the circuit balance is good, so there is little fundamentalLO signal at the common emitters of the switching quad. In addition, theleakage across the base-collector capacitors is small, so the LO-to-RFisolation obtained from the double-balanced Gilbert mixer is excellent.However, it has been found that at 60 GHz (and other millimeter-wavefrequencies), circuit balance is not as good, because the transistorsmust be made small and biased at high current densities, and thereforewill not match as well as larger transistors biased at lower currentdensities. In addition, significant leakage can occur across even smallbase-collector capacitances, because the frequencies are so high.Consequently, LO-to-RF leakage through this circuit can becomesignificant at 60 GHz.

In accordance with a preferred embodiment of the present invention, andwith reference to FIG. 2, the leakage of the LO fundamental frequencythrough the base-collector capacitances can preferably be reduced by theaddition of cascode transistors Q8 and Q9. Indeed, cascoding is a wellknown technique for reducing the feedback across the base-collectorcapacitance of a transistor, often used to improve the performance of anamplifier at high frequencies. However, it is not known that such atechnique has been used for improving mixer isolation.

Although Q8 and Q9 in FIG. 2 significantly improve LO-to-RF isolation,the additional transistor in the stack can reduce the voltage headroomin the circuit. Accordingly, in accordance with a preferred embodimentof the present invention, this can be remedied by removing theconstant-current source transistor Q7 in FIG. 2 and replacing it with aparallel resonant LC circuit, as shown in FIG. 3. This results in thecircuit having a high effective AC impedance at the frequency ofinterest (in this case, 60 GHz), but without the DC voltage dropassociated with an active constant-current source.

By way of experimentation, circuit simulations were run to quantify theamount of improvement in LO-to-RF isolation that occurs with theaddition of the cascode transistors; results of the simulations areshown in FIG. 5. Accordingly, the circuit in FIG. 3 was simulated bothwith (FIG. 3) and without (FIG. 4) cascode transistors Q8 and Q9. Theswitching transistors Q3-6 were imbalanced by increasing the emitterarea of Q3 by 20% (from 2×0.12 μm² to 2.4×0.12 μm²) to model theimperfect transistor matching that occurs when small-area devices arebiased at high current densities. Such a circuit imbalance increases theLO-to-RF leakage at the fundamental frequency of 60 GHz because of theimperfect cancellation of the fundamental at the common emitters of Q3-4and Q5-6. In simulation, then, the addition of Q8 and Q9 reduced theleakage of a 60 GHz LO signal to the RF input from 663 μV to 153 μV, orby 12.7 dB.

Overall, although many variations on a Gilbert mixer have been presentedheretofore [3-8], nothing on the order of the arrangements presented inFIGS. 2 & 3 is known to have been hitherto realized. While the use of acascode device in the input stage of a Gilbert mixer is shown in FIG. 12of Gilbert [10], it is used solely for the improved biasing of anunusual class-AB input stage, which is completely different from theembodiments of the present invention (e.g. as illustratively andnon-restrictively exemplified in FIGS. 2 & 3).

Generally, it is broadly contemplated that circuits in accordance withat least one embodiment of the present invention be realized in aSilicon-Germanium BiCMOS process (IBM BiCMOS8HP), as was done in thesimulation mentioned heretofore. However, it should be appreciated andunderstood that circuits could alternatively be implemented in any of awide variety of other processes, such as in a silicon CMOS process, orin various III-V semiconductor processes, such as GaAs.

If not otherwise stated herein, it is to be assumed that all patents,patent applications, patent publications and other publications(including web-based publications) mentioned and cited herein are herebyfully incorporated by reference herein as if set forth in their entirelyherein.

Although illustrative embodiments of the present invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that the invention is not limited to those preciseembodiments, and that various other changes and modifications may beaffected therein by one skilled in the art without departing from thescope or spirit of the invention.

REFERENCES

-   -   1. U.S. Pat. No. 3,241,078    -   2. U.S. Pat. No. 3,689,752    -   3. U.S. Pat. No. 4,322,688    -   4. U.S. Pat. No. 5,311,086    -   5. U.S. Pat. No. 5,589,791    -   6. U.S. Pat. No. 6,054,889    -   7. U.S. Pat. No. 6,140,849    -   8. U.S. Pat. No. 6,348,830    -   9. B. Gilbert, “A Precise Four-Quadrant Multiplier with        Subnanosecond Response”, IEEE JSSC, vol. SC-3, no. 4, pp.        365-373, December 1968.    -   10. B. Gilbert, “The Micromixer: A Highly Linear Variant of the        Gilbert Mixer Using a Bisymmetric Class-AB Input Stage”, IEEE        JSSC, vol. 32, no. 9, pp. 1412-1423, September 1997.

1. A Gilbert mixer circuit comprising: at least one cascode element forimproving mixer isolation; a transconductor portion; and a switchingportion; said at least one cascode element being generally connectedbetween said transconductor portion and said switching portion; whereinsaid switching portion comprises a switching quad; wherein saidswitching quad comprises a first common emitter and a second commonemitter; wherein said first common emitter and said second commonemitter each comprise a LO signal input; wherein said at least onecascode element comprises a pair of cascode transistors; wherein saidcircuit further comprises a pair of RF signal inputs; and wherein saidpair of cascode transistors is respectively connected between one ofsaid LO signal inputs and one of said RF signal inputs, whereby LO-to-RFisolation is improved.
 2. The circuit according to claim 1, wherein saidpair of cascode transistors comprise bipolar transistors.
 3. The circuitaccording to claim 1, wherein said pair of cascode transistors compriseMOSFETs, MESFETs, or HEMTs.
 4. A method of improving mixer isolation ina Gilbert mixer, said method comprising: providing at least one cascodeelement for improving mixer isolation; providing a transconductorportion; providing a switching portion; generally connecting the atleast one cascode element between the transconductor portion and theswitching portion; and providing a pair of RF signal inputs; whereinsaid step of providing a switching portion comprises providing aswitching quad; wherein said step of providing a switching quadcomprises providing a first common emitter and a second common emitter;wherein said step of providing a first common emitter and a secondcommon emitter comprises providing respective LO signal inputs; whereinsaid step of providing at least one cascode element comprises providinga pair of cascode transistors; and wherein said step of providing a pairof cascode transistors comprises respectively connecting each of thecascode transistors between one of the respective LO signal inputs andone of the RF signal inputs, whereby LO-to-RF isolation is improved. 5.The method according to claim 4, wherein said step of providing atransconductor portion comprises configuring the transconductor portionto employ resistive degeneration for increasing linear input signalrange.
 6. The method according to claim 4, further comprising the stepof providing a parallel resonant LC circuit which provides biasing. 7.The method according to claim 4, wherein said step or providing a pairof cascode transistors comprise providing bipolar transistors.
 8. Themethod according to claim 7, wherein said step of providing atransconductor portion comprises configuring the transconductor portionto employ inductive degeneration for increasing linear input signalrange.
 9. The method according to claim 7, wherein said step ofproviding a transconductor portion comprises configuring thetransconductor portion to employ resistive degeneration for increasinglinear input signal range.
 10. The method according to claim 7, furthercomprising the step of providing a parallel resonant LC circuit whichprovides biasing.
 11. The method according to claim 7, wherein said stepor providing a pair of cascade transistors comprise providing bipolartransistors.
 12. The method according to claim 7, wherein said step ofproviding a pair of cascade transistors comprises providing MOSFETs,MESFETs, or HLMTs.